Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when a window panel is being directed to the closed position. Such sensors can also be used to detect the presence of obstructions in other types of automotive closures such as sunroofs, side doors, sliding doors, lift gates, and deck lids.
A variety of capacitor-based proximity sensors are known in the art. For example, U.S. Pat. No. 6,377,009 discloses a system for preventing the pinching or trapping of a foreign object by a closing panel (such as a window) through the use of a sensing electrode or plate. This electrode is a metal strip or wire which is embedded in a plastic or rubber molding strip placed behind a piece of fascia or other trim part. The metal strip or wire and the chassis of the vehicle collectively form the two plates of a sensing capacitor. A foreign object placed between these two electrodes changes the dielectric constant and thus varies the amount of charge stored by the sensing capacitor over a given period of time. The charge stored by the sensor capacitor is transferred to a reference capacitor in order to detect the presence of a foreign object. Similar capacitive sensing applications are known from DE 4036465A, DE 4416803A, DE 3513051A1, DE 4004353A.
There are two major problems that have to be overcome for capacitive anti-pinch systems to work well in practice.
The first problem relates to the large background capacitance presented by the relatively enormous area of the sheet metal and plastic surrounding the closure aperture. For instance, in a power sliding door application, there is a large gap in between the sliding door and the vehicle frame. The presence of a small element such as a child's finger may not make an appreciable difference to the overall capacitance, and thus may be rejected as noise. Alternatively, if a relatively high sensitivity is employed to detect such a small change, too many false positives may occur (it being understood that no system is perfect and that there many some acceptable degree of false positives).
The second problem relates to the variable capacitance presented by changing humidity or water levels. The existence of high humidity or water will increase the dielectric constant of the system and thus will either mask the presence of a small object such as a child's finger or cause too many false positives.
In order to deal with such issues, it is known to utilize capacitive shielding and a differential capacitance sensing system which reduces the effect of parasitic capacitance arising from the sheet metal. It is also known to map the background capacitance as the closure panels opens and use that map as a reference as the closure panel closes to detect a differential. And it is known to vary the sensitivity of the system as the closure panel nears its final closing position. See, for instance, Applicant's PCT Publication Nos. WO 2002/101929, WO 2002/012699, WO 2003/038220, and WO 2005/059285.
However, the presence of water can still cause too many false positives, particularly when the sensor itself is wet. And since a human being's dielectric constant is similar to the dielectric constant of water, there could be a situation when the presence of water on the sensor causes too many false positives.